The basic methods used in the past to apply a wear resistant layer to metal substrates, such as the wear surfaces of tools and the like adapted to contact an abrasive medium have hitherto utilized autogenous welding processes or electric arc welding techniques. The welding rod or welding wire is so constructed that it has the desired composition of the wear-resistant layer and is consumed in a deposition welding operation in the formation of the coating on the surface.
In the autogenous welding process, welding additives are employed so that after the formation of the deposit weld, i.e. the weldment, tungsten carbide is found as inclusions in an iron-alloy matrix or in a nickel-alloy matrix.
The welding rod or welding wire in these systems generally comprise an iron tube which is filled with tungsten carbide grains or particles. The tungsten carbide which is employed can be a granulated tungsten-fused carbide product or tungsten carbide/cobalt pellets. The degree of filling of the iron tubes is about 80%.
It is also known in conjunction with an autogenous welding process to operate with tubes composed of an iron/chromium/nickel alloy which are filled in the aforedescribed manner with tungsten particles. The product of this process includes a matrix in the form of a corrosion resistant alloy and in which the tungsten carbide inclusions are present.
If one replaces the iron tubes by nickel tubes and fills the nickel tubes with tungsten carbide grains along with other alloy elements which are to be present in the matrix, in finely divided form, one can obtain a wear resistant layer having, for example, a nickel/chromium/boron/silicon matrix in the form of a corresponding alloy by the autogenous deposition technique.
In the electric arc welding process for applying wear resistant coatings, one can operate with rod electrodes or solid wire electrodes utilizing unalloyed iron tubes which are filled in the aforedescribed manner (compare German Utility model DE-GM No. 87 16 743, German Patent DE-PS No. 24 40 995 and U. S. Pat. No. 2,137,471).
The electric arc welding technique utilizing consumable electrodes to form a coating having a nickel-alloy matrix in accordance with prior art teachings have not, however, been found to be satisfactory heretofore.
Investigations have shown that the alloy composition of the nickel-alloy matrix has a variety of influences on the structure and the degree of solubilization of the tungsten carbide contained in the electrode. As a consequence, the structure of the tungsten carbide inclusions in the welding deposit layer becomes quite undefined and is not controllable. Accordingly, reproducible coatings of tungsten carbide inclusions in a nickel-alloy matrix cannot be formed by the prior art arc welding deposition techniques to yield uniform wear resistance and reliable protection of the surfaces treated.